Whispering gallery modes (WGMs) in optical applications is typically associated with circular-path resonant cavities where light inserted into an optical WGM travels exclusively via total internal reflection, glancing off material interfaces at a near-parallel angle. In recent years, interest in WGM optical resonators has grown dramatically, particularly with respect to its “sensing” applications. Optical sensing generally uses an optical probe beam to interact with a material to be detected. This interaction between the optical probe beam and the material modifies some aspect of the optical probe beam. A portion of this modified beam, such as the scattered light, may be collected and measured to obtain certain information associated with the material. For example, the optical intensity, phase, spectrum, polarization and/or direction of the collected light may be measured either individually or in combination with other parameters to determine the composition of the material.
Recently, the optical probe for such applications has taken on the form of a microsphere formed at the termination of an optical fiber taper. Such a microsphere can possess extremely high quality (O) factor WGMs, corresponding to narrow spectral resonances. The position and width of these resonances are extremely sensitive to changes in the ambient medium. Therefore, when used as a “sensor”, a microsphere probe may be immersed into the medium under test to determine its composition with extreme precision. U.S. Pat. No. 6,490,039 issued to L. Maleki et al. on Dec. 3, 2002, entitled Optical Sensing Based on Whispering-Gallery-Mode Microcavity, is exemplary of one such sensing arrangement that utilizes a microsphere cavity “whispering gallery mode” resonator to detect a minute amount of a particular material in a given specimen. U.S. Pat. No. 6,922,497 issued to A. Savchenkov et al. on Jul. 26, 2005, entitled Whispering Gallery Mode Resonators Based on Radiation-Sensitive Materials, discloses an alternative arrangement, using a pair of WGM optical resonators that are disposed in a cascaded configuration to allow for “tuning” of the sensing function.
A study of the prior art, however, yields the result that the use of a microsphere resonant probe has been limited to performing compositional analysis of a “medium under test”.